Laser peening of components of thin cross-section

ABSTRACT

The properties of a metal piece are altered by laser peening the piece on the first side using an acoustic coupling material operatively connected to the second side and subsequently laser peening the piece on the second side using an acoustic coupling material operatively connected to the first side

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a division of application Ser. No. 09/771,179filed Jan. 25, 2001 entitled “Laser Peening of Components of ThinCross-Section.”

[0002] The United States Government has rights in this inventionpursuant to Contract No. W-7405-ENG-48 between the United StatesDepartment of Energy and the University of California for the operationof Lawrence Livermore National Laboratory.

BACKGROUND OF THE INVENTION

[0003] 1. Field of Endeavor

[0004] The present invention relates to industrial applications oflasers and more particularly to laser peening of metals.

[0005] 2. State of Technology

[0006] Improving the strength of metals by cold working undoubtedly wasdiscovered early in civilization, as ancient man hammered out hisweapons and tools. Since the 1950s shot peening has been used as a meansto improve the fatigue properties of metals. Another method of shockprocessing involves the use of high explosive materials in contact withthe metal surface. The use of high intensity laser outputs for thegeneration of mechanical shock waves to treat the surfaces of metals hasbeen well known since the 1970s. The laser shock process can be used togenerate compressive stresses in the metal surfaces adding strength andresistance to corrosive failure. Lasers with pulse outputs of 10 to 100J and pulse durations of 10 to 100 ns are useful for generatinginertially confined plasmas on the surfaces of metals. These plasmascreate pressures in the range of 10,000 to 100,000 atmospheres and theresulting shock pressure can exceed the elastic limit of the metal andthus compressively stress a surface layer as deep or deeper than 1 mm inthe metals.

[0007] The article, “Blasts of Light to Strengthen Metals,” Science &Technology Review, October 1998, provides the following description: “Atfirst glance, it would seem that bombarding a metal part with an intensestream of tiny metal or ceramic balls might not be the best approach formaking that part more resistant to cracking and corrosion. And yet, shotpeening is a tried-and-true technique for strengthening metals. Now ateam of Lawrence Livermore researchers, in tandem with colleagues at NewJersey-based Metal Improvement Co. Inc., have replaced the tiny ballswith short-lived, repetitive blasts of light from a reliable,high-powered laser.”

[0008] U.S. Pat. No. 4,401,477 for laser shock processing, by Clauer etal, patented Aug. 30, 1983, provides the following description “. . .constitutes an improvement of the U.S. Pat. No. 3,850,698 which is thebasic method of laser shock processing a solid target with a radiationpulse of high power generation, the disclosure of which is hereinincorporated by reference. The patented invention is particularly usefulwhen the target material is sufficiently thick so that the stress wavedoes not penetrate to the back surface of the target material. Theirmore recent invention involved “the mounting of a trapping material(hereinafter referred to as a momentum trap) that is placed along theback surface of the metal substrate to be laser shock processed. It isimportant that the trap and the substrate have substantially the sameacoustical impedance. Other material properties that are important are amatching of material densities, and sound speed. Preferably, the trapand the substrate are the same material, so that no matching is needed.The trap is placed against the back surface of the substrate, andmineral oil can be used there between to enhance the coupling of thematerials. A spring-loaded, disk-shaped trapping material is placedagainst the substrate. An overlay material that is substantiallytransparent to laser radiation (e.g.—fused quartz, acrylic, water) isplaced in direct contact with the front surface of the specimen.Initially the specimen is coated with a thin layer of paint, preferablyblack, to enhance absorption of the laser radiation and to protect thesurface from melting by the radiation. A high power, pulsed laser emitsa high intensity pulse of short duration radiation that passes throughthe overlay material, is absorbed by the paint, produces a high pressurecondition at the front surface of the metal substrate, thereby driving ahigh amplitude stress wave into and through the metal substrate and intothe momentum trap. The surface of the paint is vaporized when it isstruck by the laser radiation causing an extremely high pressure nearthe surface when the vaporized gas is trapped between the specimensurface and the transparent overlay. At the back surface of the targetspecimen the stress wave passes into the momentum trap and is reflectedback from the back surfaces. However, the momentum imparted to the trapby the reflected wave causes the spring loaded trap to break away anddisconnect from the specimen after the stress wave is reflected from theback surface of the momentum trap and strike the common surface of thespecimen and the trap. The momentum trap thereby carries away thetensile wave which would produce distortion in the specimen. Anothermeans of absorbing the stress wave after it has passed through the metalsubstrate involves the use of a large mass of material, having a longdimension in the same direction as the stress wave, and havingsubstantially the same acoustical impedance as the metal substrate.Being in direct contact with the metal substrate, and mounted so thatthe stress wave passes through the long dimension of the material mass,the stress wave is considerably weakened when it is reflected back tothe common surface of the mass and the substrate. Copending patentapplication Ser. No. 334,612, now European Patent No. 085278A1 for splitbeam method of altering material properties, by Clauer et al, issuedAug. 10, 1983, discloses the use of a split beam to simultaneously lasershock the opposing two sides of the target material. However, the splitbeam method has several inherent disadvantages. 1. To shock process twosides simultaneously requires that the laser generate twice the power asfor only doing one side. High powered lasers are extremely expensive andsuch cost may be prohibitive. 2. The split beam method requires the useof precisely calibrated and positioned lenses and mirrors. Thisprecision may be difficult to achieve in a high production manufacturingenvironment. 3. In many applications only one side of the metalsubstrate is subject to fatigue, so there is no need to use the splitbeam method. In other applications, it is not possible to have line ofsight access to both sides of the fatigue critical region for split beamprocessing.” This description of the process for simultaneous dual sidedpeening thin sections fails to adequately recognize or address theproblem of internal spalling.”

[0009] U.S. Pat. No. 5,531,570 for distortion control for laser shockpeened gas turbine engine compressor blade edges, by Mannava et al,patented Jul. 2, 1996, provides the following description, “A method forcounteracting distortion of the airfoil caused by laser shock peening agas turbine engine compressor metallic airfoil along its leading and/ortrailing edge to form laser shock peened surfaces extending radiallyalong at least a portion of the edges with a region having deepcompressive residual stresses imparted by laser shock peening (LSP)extending into the airfoil from the laser shock peened surfaces.”Although this patent discusses distortion of the part shape, it alsofails to address an effective approach to peening of thin componentswithout internally spalling.

SUMMARY OF THE INVENTION

[0010] The present invention provides a system for altering theproperties of a metal piece having first and second surfaces by aprocess of laser peening. In an embodiment of the invention, theproperties of a metal piece having first side and a second side arealtered by laser peening the piece on the first side using an acousticcoupling material operatively connected to the second side. In anotherembodiment of the invention, laser peening is employed from one side ata time with a material on the back side that couples a portion or all ofthe shock wave out of the metal. Other features and advantages of thepresent invention will become apparent from the following detaileddescription. It should be understood, however, that the detaileddescription and the examples, while indicating specific embodiments ofthe invention, are given by way of illustration only, since variouschanges and modifications within the spirit and scope of the inventionwill become apparent to those skilled in the art from this detaileddescription and by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The accompanying drawings, which are incorporated into andconstitute a part of the specification, illustrate embodiments of theinvention and, together with the general description of the invention,and the detailed description of the embodiments, serve to explain theprinciples of the invention.

[0012]FIG. 1 shows a system for laser peening a part.

[0013]FIGS. 2A and 2B illustrate the behavior of the shock waves fordual sided and single sided peening.

[0014]FIG. 3 shows a counterbalancing effect.

[0015]FIG. 4 shows rectangular pulses systematically applied next toeach other with just overlapping edges.

[0016]FIG. 5 shows a system for laser peening a part.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Referring now to the drawings, specific embodiments of theinvention are illustrated. The detailed description of the specificembodiments, together with the general description of the invention,serve to explain the principles of the invention.

[0018] Improving the strength of metals by cold working undoubtedly wasdiscovered early in civilization, as ancient man hammered out hisweapons and tools. Since the 1950s shot peening has been used as a meansto improve the fatigue properties of metals. At first glance, it wouldseem that bombarding a metal part with an intense stream of tiny metalor ceramic balls might not be the best approach for making that partmore resistant to cracking and corrosion. And yet, shot peening is atried-and-true technique for strengthening metals. The use of highintensity laser outputs for the generation of mechanical shock waves totreat the surfaces of metals has been well known since the 1970s. Thelaser shock process can be used to generate compressive stresses in themetal surfaces adding fatigue strength and resistance to corrosivefailure.

[0019] The currently accepted practice for laser peening thin sectionmaterials, such as jet engine fan and compressor blades, is to apply asimultaneous laser pulse to each side of the piece being peened. Thisapproach results from the belief that the simultaneous counterpropagating shock waves will counteract the distortion associated withpeening only one side at a time. However one can recognize that thesimultaneous waves actually create a deleterious doubly intense tensilewave within the part. This intense tensile wave can undo the peeningeffect and actually rupture (spall) the interior material. The presentinvention employs laser peening from one side at a time and places amaterial on the back side that couples the shock wave out of the metalpreventing its reflection and deleterious effect.

[0020] Referring now to FIG. 1, a system for laser peening a part isshown and designated generally by the reference numeral 10. The nearfield output of a beam from a laser such as a Nd:glass laser is imagerelayed to the part to be peened. The front surface of the part iscoated with an ablative layer and a pressure confinement (tamping) layerof fluid is flowed over the ablative layer. This layer, transparent tothe laser light, confines the plasma pressure that develops and greatlyincreases the intensity of the shock wave that transmits into the metal.In the recommended configuration, the laser beam output comprises arectangular beam that allows precise and uniform overlap of pulses andhence highly uniform compressive stress from front to back side withinthe part. This minimizes distortion of the peened part.

[0021] It has become accepted practice in the laser peening of metalcomponents with thin cross sections to simultaneously peen both sideswith the intent of preventing component distortion. This technique isdescribed in a European patent European Patent Application 82810554.4filed Dec. 20, 1982 entitled “Split Beam Method of Altering MaterialProperties” by Clauer et al and U.S. Pat. No. 5,531,570 entitled“Distortion Control for Laser Shock Peened Gas Turbine Engine CompressorBlade Edges” by S. R. Mannava et al of General Electric Company.

[0022] It was believed that by bringing a split laser pulse to each sideof the metal, nearly equal opposing forces and hence traveling shockwaves could be generated to prevent “back-side” spallation or distortionof the metal. In the analysis that led to the present specificembodiments, it was learned that the propagation duration of the shockwave through the metal is substantially long, on the order of 200 ns,such that the typically 20 ns long duration laser pulse and itscorresponding back pressure have long since ended before the shock wavereaches the output surface. Thus the intended effect of balancing theforces will not be realized. In fact, an adverse effect actually occursin that as a shock wave reaches the metal to air (or plastic) interfaceat the far side of the part, it strongly reflects due to the impedancedifference at the boundary and additionally the reflected shock wave isinverted becoming a tensile wave. The tensile waves resulting from eachof the two reflected waves interfere near the center of the metal partcausing a doubly intense cavitation force that can spall or split apartthe metal in the center.

[0023] In the analysis that led to the present specific embodiments, itwas discovered that improvement could be made by laser peening only oneside of the thin section material at a time. A uniformly controlled,rectangular shaped laser pulse is incident from one (front) side onlyand creates a shock wave. On the back side a liquid such as water orfluorinert is flowed over the surface directly behind the area to bepeened with a thickness of about 1 mm. Alternately a sound absorbingmaterial can be pressed against the surface. Using the density of wateras 1000 kg/m3 and the sound velocity as 1461 m/s, as the shock wavepropagates to the back side a reflected wave is generated of intensityrelative R=−0.84 for water.

[0024] As indicated by the negative sign, this wave propagates back as atensile wave but because the thin material is being peened on only asingle side, it now has no counter propagating wave with which tointerfere. Thus the magnitude of the tensile force is reduced by afactor of 2.4 over that of dual sided peening, a factor that can becritical in preventing cavitation or spalling in the material.

[0025] Referring again to FIG. 1, The system 10 for laser peening a partis shown. The near field output of a beam from a laser 11 is imagerelayed by the optical imaging system 12 to the part 13 to be peened.The front surface 14 of the part 13 is coated with an ablative layer 16and a pressure confinement (tamping) layer 17 of fluid is flowed overthe ablative layer 16. The pressure confinement (tamping) layer 17 offluid is provided by the tamping fluid source 22. This layer 17,transparent to the laser light 18, confines the plasma pressure thatdevelops and greatly increases the intensity of the shock wave thattransmits into the metal part 13. The part 13 is peened with an acousticcoupling material, shock absorbing layer 19. The acoustic couplingmaterial, shock absorbing layer 19 can be a material such as water orfluorinert applied over the back side 15 of the part 13. The acousticcoupling material/shock absorbing layer 19 material is provided by theshock absorber fluid source 20. The laser beam output comprises arectangular beam that allows precise and uniform overlap of pulses andhence highly uniform compressive stress from front to back side withinthe part. This minimizes distortion of the peened part. The part 13 ispositioned within the system 10 by the part manipulator 21.

[0026] The behavior of the shock waves for dual sided and single sidedpeening is illustrated graphically in FIGS. 2A and 2B. The behavior ofthe shock waves for dual sided peening, designated generally by thereference numeral 30, is illustrated in FIG. 2A. The behavior of theshock waves for single sided peening, designated generally by thereference numeral 40, is illustrated in FIG. 2B.

[0027] As shown in FIG. 2A, laser beams 31 and 32 are simultaneouslydirected onto sides 35 and 36 respectively. The compressive pressurepulse T1, for example [1], from one side 35 has traversed to theopposite side 36 by the time the first pressure pulse T1[2] reaches itsfar side 36. Both pulses reflect from their respective back sides andform reverse traveling tensile waves. These waves constructivelyinterfere near the middle of the material and form the doubly intensetensile force that can spall (tear apart) the metal in the center. Thereis also no apparent benefit to having a pressure pulse simultaneous onboth sides.

[0028] As shown in FIG. 2b, a laser beam 41 is directed onto one side 45of a part 47. The strain wave 43 propagates in from side 45. Thecompressive pressure pulse T1 from side 45 propagates from side 45. Theanalysis that led to the present specific embodiments focused oneliminating the dual counter propagating waves shown in FIG. 2A. In thepresent specific embodiments shown in FIG. 2B, the part 47 is peened onone side at a time with the acoustic coupling material 42, such as wateror fluorinert, applied over the back side 46. The closer the acousticimpedance, given by the product of material density and speed of soundin the material, of the backing is to that of the material being peened,the greater fraction of the shock wave is coupled out. After peening asingle layer of one side of a component, the component is rotated 180degrees and the opposite side is peened again with a coupling materialapplied to the back side to couple out the acoustic shock wave. Withcareful placement of the peening to each side thereby producing equalcounterbalancing compressive residual stress fields, distortion inducedfrom peening the first side is fully removed.

[0029] The counterbalancing effect is illustrated in FIG. 3. A part 57is shown at three different times in the process in FIG. 3. The part 57shown in the middle view 52 is peened on the first side 54. As show inthe views 51 and 52, peening an area 58 on one side of a thin sectionmaterial will result in distortion curvature because of the residualcompressive stress. As shown in view 53, peening on the first side 55 ofa thin section material resulted in distortion curvature because of theresidual compressive stress, but after peening in an equivalent stress59 to the back side 56, the part will return to near straight condition.

[0030] This embodiment employs only laser peening from one side at atime and places a materiel on the back side that couples the shock waveout of the metal preventing its reflection and deleterious effect. Italso uses a rectangular beam that allows precise and uniform overlap ofpulses and hence highly uniform compressive stress from front to backside within the part. In treating an entire piece, using this uniformbeam, one approach is to peen the desired area with a single layerplacing the rectangular spots precisely adjacent to each other. Afterthis first step the piece will distort in a convex shape with respect tothe direction of laser peening. Next, the piece is flipped around andthe reverse side peened with a matched layer generating an identicalcounterbalancing compressive residual stress field. As this laserpeening is completed, the distortion is essentially removed. To impartmultiple laser peening layers, the process can be repeated, by startinglaser peening on the second side and positioning the individual laserpeening spots so that the center of an individual spot coincides withthe four-corner area of the previous laser peening pattern. After thesecond side is peened, the part is reversed and the second area ispeened in a like manner.

[0031] In order to laser peen both sides of the thin section componentwith minimal distortion, several different approaches can be taken withrespect to placing the individual pulses that comprise to overalltreatment pattern. As discussed in the above referenced patents,previous workers have had difficulty with distortion believed to beprimarily associated with the circular nature of their laser spot andits non-uniform intensity profile. This causes the generation ofnon-identical compressive residual stress fields on either side of thepart, thus imparting distortion to the part's geometry.

[0032] Co-pending U.S. patent application Ser. No. 09/133,590, filedAug. 13, 1998, entitled “Laser Beam Temporal and Spatial Tailoring forLaser Shock Processing,” (the disclosure of which is incorporated hereinby reference) describes an effective means for delivering highly uniformrectangular spots that provide excellent local uniformity. The U.S.patent application Ser. No. 09/133,590 invention is a method forformatting the laser spatial shape and for effectively and efficientlydelivering the energy to a work surface in the laser shock process. Anappropriately formatted pulse helps to eliminate breakdown and generateuniform shocks. The invention uses a high power laser technology capableof meeting the laser requirements for a high throughput process, thatis, a laser which can treat many square centimeters of surface area persecond.

[0033] Another means for delivering highly uniform rectangular spots isdescribed in U.S. Pat. No. 5,689,363, entitled Long-Pulse-Width NarrowBandwidth Solid State Laser. U.S. Pat. No. 5,689,363 is incorporatedherein by reference. According to an embodiment disclosed therein, alaser head with rectangular slab amplifier comprises a laser system thatemits a rectangular shaped beam with near diffraction-limited divergenceand near transform-limited bandwidth. The system consists of one or moreflashlamp-pumped Nd:glass zig-zag amplifiers, a very low threshold SBSphase conjugator system, and a self seeded single frequency Nd:YLFmaster oscillator. Completely passive polarization switching provideseight amplifier gain passes. Multiple frequency output can be generatedby using SBS cells having different pressures of a gaseous SBS medium ordifferent SBS materials. This laser system produces a long pulse, lowdivergence, narrow-bandwidth, multi-frequency output. Because of itshigh average power and high beam quality, this system has application inany process which would requires a long pulse format, including thematerial processing application of the present invention.

[0034] Referring now to FIG. 4, a component 62 being peened is shown anddesignated generally by the reference numeral 60. The component ishandled by a part gripper and motion controller 65. A laser beam 61 isdirected onto the component 62 being peened. The laser beam 61 producesa spot 63 on the component being peened. The peened area 64 is produced.As shown in FIG. 4, the rectangular pulses are systematically appliednext to each other with just overlapping edges. This greatly minimizesdistortion of the peened part because a highly uniform and highlyrepeatable stress is applied to each side of the part.

[0035] This laser peening or laser shock processing process isespecially valuable for jet turbine engines. FAA Service DifficultyReports show that there are an average of 15 uncontained rotor failuresper year in the US involving turbine powered aircraft. Turbine engineuncontainment events were identified as the top priority by theAerospace Industries Association Continued Airworthiness AssessmentMethodoligies Report.

[0036] Laser peening or laser shock processing of the leading edge ofturbine blades has been clearly shown to halt the growth of cracks andenormously reduce blade failure. The leading edges of these titaniumalloy blades are 1 to a few millimeters in thickness and must becarefully treated in the laser peening or laser shock processing processto avoid damage and distortion. In the process developed and patented bypersonnel of LSP, Inc., of Dublin, Ohio, (European Patent Application82810554.4 filed Dec. 20, 1982 entitled “Split Beam Method of AlteringMaterial Properties” filed by Clauer et al) and a similar processpatented by General Electric Aircraft Engines of Cincinnati, Ohio (U.S.Pat. No. 5,531,570 “Distortion Control for Laser Shock Peened GasTurbine Engine Compressor Blade Edges” by S. R. Mannava et al of GeneralElectric Company) the simultaneous peening or laser shock processing ofboth sides of the blade is described as the means to reduce distortion.As described in the GE patent, “the blade may have laser shock peenedsurfaces on both suction and pressure sides of the blade wherein bothsides were simultaneously laser shock peened. The process is performedwith circular shaped laser beams which is a consequence of the previouslaser design and makes uniform overlap of the peened areas difficult.The inventors/authors describe “circular laser shocked areas such thatthe suction side pattern is different from the compression side patternand each of the patterns includes overlapping rows of circular lasershocked areas.” The simultaneous peening or laser shock processing ofboth sides of the blade and the use of varying overlap of the circularpatterns are described in the GE patent.

[0037] In the process, laser light of typically 100 to 200 J/cm2 passesthrough a confining layer (typically 1 mm thickness of water) and isincident on an ablation layer (typically a plastic of a few hundredmicron thickness) to create a high pressure shock wave. Although thelaser pulse lasts for only 20 ns, the shock wave propagates through theblade at acoustic sound speed which is approximately 4000 meters persecond for titanium 6-4 alloy. In order to travel the blade thickness of1 mm to 2 mm requires 250 ns to 500 ns. For a laser beam incident fromeach side simultaneously, the laser pulse and plasma pressure are welldissipated by the time the shock wave reaches the opposite side,hundreds of nanoseconds later. The plasma pressure of about 70 kbar thatformed on each side during the incident pulse has lifted the watercovering layer off the surface by approximately 40 microns. Now thebackside has just the ablation layer which is a poor acoustic match tothe metal and beyond it a void left by the displaced water layer. Withthis interface situation, the acoustic shock wave will mostly reflect asa tensile wave back into the metal. The intensity of the reflectedstress wave will be given by the formula (see “Behavior of Metals UnderImplusive Loads” by John s. Rinehard and John Pearson, page 42, DoverPublications, Inc., New York 1954);

R=(ρ₁ c ₁−ρ₁ c ₁)/(ρ₁ c ₁−ρ₁ c ₁)

[0038] Where ρ₁ is the metal density, ρ₂ is the density of the materialjust across the boundary (one atmosphere of air or the impedance of theremaining ablation layer since the pressure pulse has dissipated), c₁and c₂ are the velocities of sound in the respective media. For Titaniumwith a density of 4500 kg/m3 and a sound velocity of 4000 m/s and airwith a density of 1.29 kg/m3 and a sound velocity of 340 m/s thereflected intensity is R=−1 where the minus sign indicates a tensilereflected wave.

[0039] For dual sided peening, a strong tensile wave reflects from bothsides of the metal/air interface and can overlap in the center forming atensile wave of twice the incident stress. This strong tensile stresscan cause cavitation and spalling within the metal. For air or the lowdensity ablation layer the reflectivity is essentially unity.

[0040] A uniformly controlled, rectangular shaped laser pulse isincident from one (front) side only and creates a shock wave. On theback side a liquid such as water or fluorinert is flowed over thesurface directly behind the area to be peened with a thickness of about1 mm. Alternately a sound absorbing material can be pressed against thesurface. Using the density of water as 1000 kg/m3 and the sound velocityas 1461 m/s as the shock wave propagates to the back side the reflectedwave is now of intensity R=−0.84 for water

[0041] This wave propagates back as a tensile wave but now has nocounter propagating wave with which to interfere. Thus the magnitude ofthe tensile force is reduced by a factor of 2.4 a factor that can becritical in preventing cavitation or spalling in the material.

[0042] Referring now to FIG. 5, an alternate approach to dual sidedlaser peening of thin sections is to peen individual spots first on thefront side and then on the backside. The structural elements of thesystem are shown. A Nd:glass Laser 71 produces laser beam 81. Thepolarization rotator 72 provides the position of ¼ polarization shifterand causes the beam to take propagate along one of two paths, eitherup/to right or horizontal/up/to left path. A polarization beam splitter79 controls the polarization of the laser 71 to be either transverseelectric or transverse magnetic causing the beam 81 to either passthrough the splitter 79 or reflect off of it. The part 73 is handled bythe motion controller 74 which moves the part 73 after 2nd, 4th, 6th, .. . pulse. The surface of the part 73 is coated with an ablative layer78 and a pressure confinement (tamping) layer 77. The polarizationrotator 72 provides for two alternate beam paths 75 and 76. One beampath 75 images the beam onto the front side 75. This side fires forpulses 1,3,7 etc. One beam path imaging the beam onto the back side 76.This side fires for pulses 2,4,6, etc. This allows local laser peeningof the front side and back side on sequential laser pulses.

[0043] To accomplish the dual sided laser peening, the single beam 81output of the laser 71 passes through a polarization rotator 72 and thenonto a polarization beam splitter 79. By controlling the polarization ofthe laser 71 to be either transverse electric or transverse magnetic,the beam 81 can be made to either pass through the splitter 79 orreflect off of it. This provides for two possible beam paths 75 and 76one imaging the beam onto the front side 75 and one onto the back side76. This allows local laser peening of the front side and back side onsequential laser pulses. The desired surface area is then treated bystepping pulses over the desired peening area while alternating betweenfront and back sides of the piece.

[0044] While the invention may be susceptible to various modificationsand alternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

The invention claimed is
 1. A metal piece, having first side and asecond side, produced by the method of: operatively connecting anacoustic coupling material to said second side of said metal piece, andlaser peening all or a portion of said metal piece from said first side.2. The metal piece produced by the method of claim 19, wherein saidlaser peening said metal piece from said first side creates a shock wavein said metal piece and wherein said acoustic coupling material couplesa portion or all of the shock wave out of said metal piece from saidsecond side.
 3. The metal piece produced by the method of claim 19,wherein said laser peening said metal piece from said first sideprocesses rectangular spots on said first side.
 4. The metal pieceproduced by the method of claim 19, wherein said laser peening saidpiece from said first side utilizes a laser that provides asubstantially rectangular spot.
 5. The metal piece produced by themethod of claim 19, including operatively connecting an acousticcoupling material to said first side of said metal piece and laserpeening all or a portion of said metal piece from said second side. 6.The metal piece produced by the method of claim 19, wherein said laserpeening said metal piece from said second side creates a shock wave insaid metal piece and wherein said acoustic coupling material couples aportion or all of the shock wave out of said metal piece from said firstside.
 7. The metal piece produced by the method of claim 24, whereinsaid laser peening said metal piece from said second side processesrectangular spots on said second side.
 8. The metal piece produced bythe method of claim 24, wherein said laser peening said piece from saidsecond side utilizes a laser that provides a substantially rectangularspot.
 9. The metal piece produced by the method of claim 19, whereinsaid acoustic coupling material is a freon compound or water.
 10. Themetal piece produced by the method of claim 19, wherein said acousticcoupling material is a freon compound.
 11. The metal piece produced bythe method of claim 19, wherein said acoustic coupling material iswater.
 12. The metal piece produced by the method of claim 19, whereinsaid acoustic coupling material is fluorinet.
 13. The metal pieceproduced by the method of claim 19, wherein said laser peening all or aportion of said metal piece from said first side includes operativelyconnecting an ablation layer of material to said first side of saidmetal piece and transmitting laser light into said ablation layer ofmaterial.
 14. The metal piece produced by the method of claim 31,wherein said laser light is transmitted into but not out of saidablation layer of material.
 15. The metal piece produced by the methodof claim 32, including operatively connecting a tamping layer to saidablation layer of material.
 16. The metal piece of claim 19, produced bythe method including the additional steps of operatively connecting anacoustic coupling material to said first side of said metal piece andlaser peening said metal piece from said second side, wherein said laserpeening is produced by laser pulses alternately directed onto said firstside and said second side, wherein sequential laser pulses are steppedover said first side and over said second side while alternating betweensaid first side and said second side.
 17. The metal piece of claim 34,wherein said sequential laser pulses are stepped over said first sideand over said second side while alternating between said first side andsaid second side on equivalent spots on said first side and said secondside.
 18. The metal piece of claim 35, wherein said equivalent spots onsaid first side and said second side are adjacent to each other.